The chief purpose of the proposed research is to document the utility of ambulatory blood pressure measurements in the assessment of arterial hypertension. It has long been recognized that conventional office blood pressure measurements are influenced by transient states of psychophysiological arousal evoked by the medical examination. Individual differences in susceptibility to this form of arousal may be responsible for the difficulty in predicting prognosis on the basis of office readings: some patients with relatively moderate pressure elevations develop hypertensive and/or atherosclerotic complications, while others with comparable elevations remain free of vascular involvement. Using a portable electronic recorder to obtain more representative samples of blood pressure during the patient's routine activities, we have shown: (a) that ambulatory pressures are substantially lower in most patients than office readings, and (b) that, when differences in office pressures are statistically adjusted, contemporaneous vascular involvement is clearly related to the level of ambulatory pressure. We now propose a prospective study in which we will attempt to perform follow-up assessments of the 1,100 essential hypertensives for whom we have collected ambulatory pressure data over the past twelve years. The data will be analyzed to determine the prognostic validity of ambulatory pressure (average level and variability) with respect to subsequent vascular disease in treated and untreated samples. Three subsidiary objectives are: (a) to study the rate of development of complications in a retrospective analysis of clinical data collected prior to the advent of modern anti-hypertensive therapy, (b) to examine the relationship between plasma renin activity and ambulatory pressure in untreated patients, and (c) to collect ambulatory blood pressure data in an age and sex stratified sample of healthy, employed industrial workers to establish norms.